This invention pertains to an improved coal liquefaction process in which particulate coal is slurried with liquid carbon dioxide to provide a flowable feedstream. It pertains particularly to a coal hydrogenation and liquefaction process in which pressurized particulate coal feed is slurried with liquid carbon dioxide, before being further pressurized and fed into a catalytic reactor for producing hydrocarbon gas and liquid products.
For known coal liquefaction processes, the coal feed in particulate form is conventionally slurried with a hydrocarbon liquid such as recycled solvent oil and then pressurized sufficiently to feed the coal/oil slurry into a catalytic reactor operated at high temperature and pressure conditions. Examples of such coal liquefaction processes are disclosed by U.S. Pat. No. 3,519,555 to Keith et al, U.S. Pat. No. 3,617,465 to Wolk et al, U.S. Pat. No. 3,700,584 to Johanson et al, and U.S. Pat. No. 4,437,973 to Huibers et al. Although such coal/oil slurrying systems have been found useful in coal liquefaction processes, they have the disadvantage of requiring the processing of undesirably large quantities of recycled oil needed only for the coal slurrying and feeding purposes. Also, a method for feeding substantially dry coal particles to a high reactor pressure by utilizing special screw feeder devices connected in series is disclosed by U.S. Pat. No. 3,775,071 to Hoffert et al. But such dry coal feeding devices have been found to be complicated and unreliable, and have not been used commercially in coal liquefaction processes.
It is also known to prepare slurries of particulate coal and water or liquid CO.sub.2 to produce relatively high coal concentrations for transfering coal through pipelines and for feeding such slurries to coal burners and combustion systems, such as used in fired boilers for generating steam. However, such combustion processes are utilized at only relatively low pressures such as 15-100 psig. These disadvantages of the prior coal/oil slurry feeding to liquefaction processes have now been unexpectedly overcome by the present invention, which provides an improved coal liquefaction process having only minimal need for recycling a coal-derived slurrying oil to the coal slurrying step in the process.